Published Apr 29, 2019 at 10:12 AM

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In this March 27, 2019, file photo, Boeing Co. 737 Max airplanes are seen at the company's manufacturing facility in Renton, Washington.

Boeing did not tell Southwest Airlines, its largest 737 Max customer, that a standard safety feature designed to warn pilots about malfunctioning sensors had been deactivated on the jets.

The safety feature is an alert that lights up in the cockpit if a plane’s angle-of-attack sensors transmit faulty data about the pitch of the plane’s nose. This feature is known as an angle-of-attack disagree light and was included in previous versions of the 737.

Southwest did not know about the change until after the fatal crash of a Lion Air flight in Indonesia. The airline, in a statement to CNBC, said Boeing had indicated through its manual that the disagree lights were functional on the 737 Max.

Southwest said Boeing told the airline that the disagree lights were inoperable only after the Lion Air crash. The airline subsequently took action to turn the alerts on.

Southwest Airline’s statement:

Upon delivery (prior to the Lion Air event), the AOA Disagree lights were depicted to us by Boeing as operable on all MAX aircraft, regardless of the selection of optional AOA Indicators on the Primary Flight Display (PFD). The manual documentation presented by Boeing at Southwest’s MAX entry into service indicated the AOA Disagree Light functioned on the aircraft, similar to the Lights on our NG series. After the Lion Air event, Boeing notified us that the AOA Disagree Lights were inoperable without the optional AOA Indicators on the MAX aircraft. At that time, Southwest installed the AOA Indicators on the PFD, resulting in the activation of the AOA Disagree lights - both items now serve as an additional crosscheck on all MAX aircraft.

The FAA even considered grounding Southwest’s Max fleet while they weighed whether or not the airline’s pilots needed additional training about the safety alert, according to the Journal. But those discussions were brief and did not go up the chain, the newspaper reported.

Boeing, in a statement to CNBC, said the “angle-of-attack” disagree lights would be included as a standard Max feature.

“As we return to service, all customers will have the AOA disagree alert as standard and have the option to include the AOA indicator at no cost,” a Boeing spokesperson said. “This change will be made to all MAX aircraft – production and retrofit.”

Boeing’s 737 Max was grounded by the FAA in March in the wake of fatal crashes in Indonesia and Ethiopia that killed 346 people.

Several major airlines have extended Max flight cancellations through the summer. American has canceled Max flights through Aug. 19, totaling 115 flights per day, while Southwest has canceled through Aug. 5 and United through June 5.